Method and apparatus for measuring electrical properties



Nm., M, 1950 H. R. LARsoN METHOD AND APPARATUS FoR MEASURTNG ELECTRICAL PROPERTIES Original Filed April 16, 1945 NiMH/77 T EN E@ i www@ Em mm EMM/fr Nov., i4, i950 H. R.

METHOD AND APP LARSON ARATUS FOR MEASURING ELECTRICAL PROPERTIES Original Filed April 16, 1945 5 Sheetsheet 2 d l i @fi-w 70@ A TMm/Hf LRSON H. METHOD AND APPARATUS FOR MEASURING ELECTRICAL PROPERTIES llllllllllllllllllllllllllllllll Hilllllllilllllllllllllllllllll Nov., 14, 1950 Original Filed April 16, 1945 wfe/Q-H/G/f #Pam/sway Gem/970,@

Patented Nov. 14, 1950 METHOD AND APPARATUS FOR MEASURING ELECTRICAL PROPERTIES Harry R. Larson, Chicago, Ill., assignor to-Weltern Electric Company, Incorporated, New York, N. Y., a corporation of New York Original application April 16, 1945, Serial No.

588,702, now Patent No. 2,491,669, dated December 20, 1949. Divided and this application March l2, 1947, Serial No. 734,287

(Cl. F15- 183) 18 Claims.

This invention relates to a method and apparatus for measuring electrical properties, and more particularly to a method and apparatus for determination of impedance in ultra-high frequency systems and is a division of my copending application, Serial No. 588,702,'fl1ed April 16, 1945, now U. S. Patent No. 2,491,669, issued December 20. 1949.

Three important reasons dictate the necessity for precise impedance matching in ultra-high frequency systems. Power transfer is at a maximum when impedances are matched. Mismatched impedances in an ultra-high frequency system will cause undesirable reflections of the waves traveling through the component parts. Reenforcement of the incident waves by these reflections may cause dielectric breakdown in the system. Reflections will also cause frequency drift and may cause the oscillator tube to completely cease functioning. It is Well known that the standing wave ratio in an ultra-high frequency system is a function of the impedance relationships in the system and that a ratio higher than unity indicates an impedance mismatch.

The existence of mismatched impedances in ultra-high frequency systems is ordinarily ascertained from standing wave observations made with what is known in the art as a traveling standing wave detector inserted into the line and electrically connected to a current indicating device, usually a microammeter, to indicate the rectied output of the detector. The rectifying element of the'detector is ir. most cases a silicon crystal. Investigation has proved that the response of the crystal does not follow a square law nor a different but constant power (exponent) law. With rare exceptions, the crystal response followsr a widely variable power law as the load on the crystal changes through the ranges involved in standing wave measurements. Neither a square law assumptionnor a calibration based on a constant exponent law for the crystal will give the precise standing wave ratio measurements so urgently necessary in the reiined micro- Lwave techniques of today, especially in radar.

Even if a careful and laborious point-by-point calibration of the crystal is made, it may change in unpredictable direction and magnitude from day to day as a result of comparatively slight 'v changes in conditions under which it is used.

Frequently changes take place for which no assignable cause has been found. It is obvious that a method of standing wave measurement which is independent of the crystalcharacteristics is not only desirable but essential iffprecise measurements are required.

Objects of this invention are to provide a method of and apparatus for very precise measurement of electrical characteristics.

In accordance with one embodiment of this invention a continuously variable radio frequency attenuator, with a rricrometric depth adjustment and a depth gage, calibrated directly in standing wave ratios or decibels, is inserted into a longitudinal slot cut into a wave guide section. The attenuating element is a rectangular piece of resistive strip half a wave length long, or if a higher degree of attenuation is desired its length may be an integral number of half-wave lengths. This length is selected to avoid phase ell'ects on the attenuation.

The attenuator is inserted in a system between an ultra-high frequency generator and a traveling standing wave detector which is connected to the unknown section under test which in turn is coupled to a totally absorbing termination. The probe of the detector is moved until the minimum current reading is indicated on the detector meter, then the probe is moved to the point of highest current. Sufcient attenuation is then introduced to bring the detector meter needle back to the minimum current value rst noted. The calibrated attenuator then yields the value of the standing wave ratio directly. y

A complete understanding of Vthe invention may be had by reference to the following description taken in conjunction with the accompanying drawings, in which Fig. 1 is aplan view of an apparatus embodying the invention and showing a method of con- 3 Fig. 6 is a central longitudinal sectional view of the attenuator taken on the -line 6--6 of Fig. 5;

Fig. 7 is` a horizontal sectional view of the attenuator taken on the line I-l of Fig. 6;

Fig. 8 is a plan view of a modified form of the apparatus showing the attenuator inserted between the probe and the crystal of the detector;

Fig. 9 is a sectional view of the apparatus shown in Fig. 8 taken on the line 3-9 of Fig. 8 with part shown in elevation, and

Fig. lo is a graphic illustration showing the relationship of the observed current curve to the cos2 curve.

The apparatus embodying the invention comprises an ultra-high frequency source of energy such as a magnetron oscillator or a velocity modulated tube generator, a movable detector for measuring the energy at different points along a wave guide, and an attenuator by means of which the energy in the wave guide may be attenuated so that the detector when set at the maximum point of a standing wave may be made to in dicate the same value as at a minimum point and the relative value of reected energy may then be determined by the amount of attenuation.

A radio frequency attenuator indicated generally at 9 is built upon a section of rectangular wave guide I0, which is provided with suitable coupling flanges II and I2 to permit the connection of the section into an ultra-high radio frequency system. Transverse leakage of power at the joints is prevented by a circular groove I3 formed into the face of the flange which acts as a radio frequency choke. A longitudinal slot I4 of proper size is cut into one of the wide sides of the guide to allow an attenuating element I5. a

half-wave length long, to be insertedv into the wave guide and retracted therefrom at will. If a higher degree of attenuation is desired, the length of the element I may be increased an integral number of half-wave lengths. The attenuating element, a rectangular piece of 200 ohms per square inch resistive strip capable of absorbing ultra-high frequency energy, is clamped with the plane of the strip parallel to the length of the wave guide, between a block I6 and a cross bar I'I having extended lugs 2| and 22 slidably engaging two vertical guide slots 23 and 24 formed in the legs of a main support member 25 having an inverted U-shaped lower' portion which is firmly mounted on two rectangular uprights 26 and 21 secured to the wave guide I0 at either end of the slot I4.

Vertical reciprocation is imparted to the attenuating element by fastening the cross bar I1 to the lower end of a vertically disposed rod 3| reciprocably supported in a bore 32 formed in the main support member 25, the lower end of the bore terminating in an aperture 33 which is reduced in diameter to guide the downwardly extending lower end of the rod. The upper end of the rod is threaded to cooperate with a thumb nut 34 which, when manually turned, will impart a gradual vertical movement to the rod either up or down, depending on the rotational direction of thethumb nut. An annular shoulder 35 on the rod 3| slidably engages the walls of .the bore 32 to firmly guide the rod. The portion of the rod extending downward from the shoulder 35 is reduced in diameter to provide room between the wall of the bore and therod for a helical' compression spring 35 which encircles the rod between the shoulder 35 and the lower end of the bore. This spring exerts a constant upward pressure against the shoulder 35 which pressure is transmitted through the rod 3| to the thumb nut III 34, the upper part of which is urged against the lower side of the horizontal portion of an inverted U-shaped gage clamping xture 31,-maintaining firmness of control and preventing backlash. Lateral displacement of the clamping fixture is prevented by tting the lower part of the legs of the clamp into vertical grooves 4I and 42 formed in the main support 25 to which the clamp is secured. To limit the verticalV position of the gage clamp 31, the legs of the clamp have shoulders 43 and 44 formed thereon to abut the top of the main support 25. Space between the top of the main support 25 and the yoke of the clamp is provided to accommodate the thumb nut 34.

Clamping pressure applied laterally by two screws 45 and 46 grips the downwardly extending bushing 49 of a dial type gage 50 in the circular enlargement of a slot 5I out lengthwise through the yoke of the clamp 31, the opposite faces of the slot operating as a vise. An actuating plunger 52 on the gage 50 passing through the bushing 49 bears against the fiat top of the rod 3| so that the gage will respond to and indicate the vertical position of the rod 3| with reference to a zero point. It is apparent that operating the thumb nut 34 will impart vertical .displacement to the rod which in turn will either lower the attenuating element I5 into the wave guide I0 through the slot I4 or retract it depending upon the direction of rotation of the thumb nut. It is further evident that the depth to which the attenuating element is lowered into the wave guide III will be indicated on the gage 5D.

The degree of attenuation introduced is a function of the depth that the attenuating element :eaches in the wave guide. The ratio between two power levels of standing waves is made a function of the degree of attenuation that is introduced to reduce the upper power level to the value of the lower. This permits the device indicating the depth of the attenuating element (in this case the dial type gage) to be calibrated directly in standing wave ratios or in decibels.

The attenuator 9 may be calibrated by coupling it between an ultra-high frequency generator 53, such as a magnetron oscillator, a velocity modulated tube generator, or the like, and Yan ultrahigh frequency detector indicated generally at 54 and described later herein. For the purpose of calibration, it is not necessary that the probe of the detector be horizontally moved,` because during the calibration standing waves are not present in the apparatus and the power level at any given instant is the same through the wave guides of the apparatus. The load characteristic of the crystal rectifier in the detector lused for calibrating the attenuator should'be accurately determined beforehand.

Since the crystal response does not follow the square law of power, its response to the various power levels must be corrected for each observa-J tion to give a true indication of the power level observed. The load characteristic of the crystal may be determined by making a traverse of approximately a wave length at the frequency at which the crystal is to be used with a standing wave detector in a line terminated with a short circuit so that the minima will be zero. The response of the crystal is observed at intervals of l millimeter and a curve 58 is plotted with the current readings representing the crystal response as ordinates and the millimetric readings of the detector position as the abscissae. The linear measure of the detector position is converted into the electrical angle at each point using the maximum point as the origin as indicated in Fig. 10. .A cos2 curve 59 having the same origin and maximum amplitude is plotted using the maximum observed current times cos2 0 as the ordinates and the o es the abscissae, 0 being the electrical angle. The corrected response of the crystal for 'any given angle will be the ordinate of the cos? curve at that angle.

A totally absorbing termination 58. hereinafter described, is coupled to the other end of tl-e standing wave detector 54. A microammeter 51 is connected to the detector 54 to measure the rectified output of'the crystal. Corresponding readings of currentand attenuator settings are made through the range of the attenuator. The current values are then corrected in accordance with the known characteristics of the crystal and the standing wave ratios or the decibels corresponding to the readings are computed and may be marked directly on the dial of the gage 50. The calibration of the attenuator will be valid indefinitely, and will thereafter be unaffected by any changes in the crystal load characteristic or by the substitution of another crystal with different load characteristics.

To determine the impedance of an unknown section 60 of an ultra-high frequency system, the

output of the radio frequency generator 53 is coupled to the input of the calibrated radio frequency attenuator 9 through a suitable transmission line or wave guide 6|. The output of the attenuator 9 is coupled to one end of a travel-.-

ing standing wave detector shown generally at 54 also supports a pair of racks 8| and 82, which in combination with pinion gears 83 and 84 actuated by a knurled knob 85 provide variable horizontal movement for the carriage block 14. Fitted into a circular aperture 86 of the carriage block is-a bushing 81 which houses a probe insulating bushing 88. The bushing 81 opens upwardly into a pickup wave guide 9| disposed at right angles to the wave guide 64, and secured to the carriage block 14.

A housing 92L for supporting the mechanism for vertically adjusting a probe shaft 93, to which the probe 1| is fixed, is mounted on the top of the pickup wave guide 9| directly over a circular aperture 94 formed in the wave guide 9|. Penetration of the probe 1| into the wave guide 64 is controlled by a vknurled nut 95. Another knurled nut 96 controls a coaxial tuner 91 to match the probe to the wave guide 9|. An independently mounted tunable crystal |0| is located in the pickup guide 9| approximately a half-wave length from a fixed back plate |02. The crystal is tuned by means of a knurled head |03 on the adjustable screw |04 mounted in a housing |05 attached to the pickup guide 9|. The

lations of the radio frequency generator 53 are transmitted through the wave guides of' the various components, passing through the attenuator guide section I0 first, then through the guide section 84 of the` detector 54, then through the unknown section 60, finally striking the termination 56. V

If the impedance of the Aunknown section matches that ofthe system, maximum power transfer will be effected and no reflection will take place, but if the impedance of the unknown section does not match the system impedance, then reflections of the original waves will be pro-v duced. The amount of reflection is dependent upon the degree of mismatch between the impedance of the-unknown section and that of the system. These reflections cause standing waves to be set up in the wave guides.l The detector probe 1| is moved horizontally by means of the .knob 85 alng the horizontal slot 61 until the minimum current is indicated on the microammeter 51 in the detector circuit and this value is noted. Next, the detector probe 1| is again moved along the slot until the maximum current isshown on the meter 51. Attenuation is then provided by turning the thumb screw 34 of the attenuator 9 which will cause theattenuating element I5 to enter the wave guide |0 through the slot |4. Suflcient attenuation is introduced to bring the needle of the meter 51 back to the minimum reading first noted. The calibrated dial gage on the attenuator 9 will thenl indicate the standing wave ratio.

The standing wave ratio is thus directly observed. Laborious andtime-consuming calculations are unnecessary, and the results are ex*- the equivalent of a full loop or loops ofthe standing wave regardless of 'its position in the guide with respect to the standing waves.

Although the method described is for use in low power systems, it can be successfully used in high power systems by, inserting the radio frequency attenuator between the probe of the standing wave detector and the rectier element of the l standing Wave detector. Between these points the power is relatively low and the attenuating element will notburn up. An embodiment of this modification of the invention herein is disclosed in Figs. 8 and 9 wherein the output of a source |2| o f ultra-high frequency oscillations, which may be a magnetron oscillator or a velocity modulated tube generator, is connected to one end of a standing wave detector, shown generally at |22, which comprises all the elements of the one indicated at 54. In addition,however, the upper suris of the same construction as the radio frequency f attenuator indicated at 9, except that the pickup wave guide |23 is substituted for the wave guide Il oi' the formerly described attenuator. A slot |3| isformed in the pickup guide |23 to receive an attenuating element |32 which may be inserted into and retracted from the guide |23 by means o! a continuously variable vertical adjustment controlled by thumb nut |33, the detail of this adjustment being the same as that oi'` the radio frequency attenuator indicated at 9. The length of the attenuating element iscne-hali wave length to prevent phase sensitivity. A microammeter |34 is connected to the output of the crystal rectifier of the standing wave detector |22 to measure the value of the rectiiied current. The other end of the detector |22 is coupled to one end of an unknown section |35 to be tested, the other end of which is coupled to a totally absorbing termination |36 which is in all respects the same as the one shown generally at l I2.

'I'his method oi impedance testing is the same as that described in the first embodiment of this i invention. Electromagnetic waves produced by the radioirequency generator |2| .pass through the wave guides of the detector |22 and the unknown section |35, striking the termination |38. If the unknown section |35 possesses a characteristic impedance that permits unopposed passage of all the power entering it, then the incident waves striking the termination |35 will be absorbed by the termination; however, should the unknown section offer opposition to maximum power transference, then the incident/waves will be reflected causing standing waves to be produced in the wave guides of the apparatus.

The probe |4| oi the detector |22 is moved horizontally by means 'of a knob |42 until a minimum current reading is indicated on the meter |34. Obviously, this will be the point Where the probe |l| intercepts the standing wave at its lowest power level. Then the probe is moved until it intercepts the standing wave at a point of maximum power; this will be indicated by maximum current reading on the meter |34. The attenuating element |32 is then lowered into the pickup guide |23 by turning the thumb nut |33 introducing suicient attenuation to reduce the value of the maximum current until the meter |34 indicates the minimum current rst noted. A calibrated gage |43 on the attenuator |26 will then indicate directly the standing wave ratio, or ii it has been so calibrated, in decibels.

What is claimed is:

1. The method of determining impedance characteristics in an ultra-high frequency system which comprises transmitting electrical energy through said system, measuring the maximum and minimum power of a standing wave in said system, attenuating the power in said system to reduce the value of the said maximum power of said standing wave to the value of said maximum power, measuring the amount of attenuation required for said reduction, and utilizing said measure of attenuation for determining impedance characteristics.

2. A method of determining standing wave ratios which comprises transmitting electrical energy through an ultra-high frequency system,

at the high point of said standing wave, attenuating the power in the system to reduce the voltage at the high point to the value of that of said low point and measuring the amount of said attenuation as a measure of the standing wave ratios.

3. A method of determining power ratios in ultra-high frequency systems which comprises transmitting high frequency wave energy throughv an ultra-high frequency system, measuring the voltage at the low point of a standing wave in :aid system, measuring the voltage at the high point of said standing wave, attenuating the power within" the system to reduce the voltage at said high point to the value of that of said low point, measuring the amount of said attenuation, and utilizing said measure or attenuation to determine the power ratio.

4. The method of determining electrical characteristics of an ultra-high frequency system l comprising transmitting electrical energy through said system, extracting and measuring a relative portion o! the power at the minimum voltage level of a standing wave in said system, extracting and measuring a. relative portion of the power at the maximum voltage level of said standing wave, attenuating the power in said system to reduce the value of the relative portion of power extracted from said maximum voltage level to the value of the relative portion of power extracted from said minimum .voltage level, and measuring the amount of said attenuation as a measure of electrical characteristics.

5. An apparatus for. measuring electrical characteristics which comprises a radio frequency generator, a radio frequency attenuator coupled to said generator, a traveing standing wave detector coupled to said radio frequency attenuator, means for measuring the value of the voltage oi a standing wave at the point of detection, a totally absorbing termination, means to connect an unknown test specimen between said traveling standing wave detector and said termination, and means on said attenuator for indicating the amount of attenuation required to reduce themaximum voltage Value determined by said measuring means at one point of detector travel to the minimum value measured at another point oi' -detector travel.

6. An apparatus for measuring electrical characteristics which comprises a radio frequency generator, a radio frequency attenuator coupled to said generator, a traveling standing wave detector coupled to said radio frequency attenuator, means for measuring the value of the voltage of a standing wave at the point of detection, a totally absorbing termination, means to connect an unknown test specimen between said traveling standing wave detector and said termination, and means for indicating the amount of attenuation required to reduce the voltage value determined by said measuring means at a point of detector travel to a lower value determined at another point of detector travel.

7. The method of measuring standing wave ratios in an electrical system which comprises detecting the low point of the voltage gradient of a standing wave in said system, indicating the extent of the detection response of said low point, detecting the high point of the voltage gradient, indicating the extent of the detection response of said high point, modifying the detection response by attenuation to reduce the high point detection response to the extent of said low point response, and measuring the amount of attenuaataque tion required to eiect said result, said amount of attenuation being a measure of the standing wa e ratio. y

8. The method of determining electrical characteristics'comprising transmitting electrical energy through an ultra-high frequency system, extracting and measuring a relative portion of the power at the minimum power level of a standing wave in said system, extracting and measuring a relative portion of the wer from the maximum power level of said stan ing wave, modifying said relative portion of the power from the maximum power level by attenuation thereby to reduce it to the value of the relative portion of power extracted from said minimum power level, and

measuring the amount of attenuation required to I effect this result as a measure of electrical characteristics.

9. The method of determining standing wave ratios in an ultra-high frequency system which comprises detecting the minimum and maximum voltage points of a standing wave in said system, measuring the voltage at said points, attenuating voltage in said system to reduce the amplitude of said maximumA voltage point to that of said minimum voltage point, and measuring the attenuation required to effect said reduction as a measure of the standing wave ratio.

10. The method of determining standing wave ratios in an ultra-high frequency system which comprises detecting the minimum and maximum voltage points of a standing wave 'in said system, measuring the voltage at such points, attenuating the power in said system to reduce the amplitude of said maximum voltage point to that of said minimumI voltage point, and measuring the attenuation required to effect said reduction as a measure of the standing wave ratio.

11. A method of determining standing wave ratios in an ultra-high frequency system which comprises extracting and measuring the value of a proportional sample of the power at a minimum Voltage point of a standing wave in said system, extracting and measuring the value of a proportional sample of the power at the maximum voltage point of said standing wave. attenuating voltage in said system to reduce the amplitude of the sample of power from said maximum voltage point to the value of the amplitude of the sample of power from said minimum voltage point, and

l t standing wave, attenuating voltage irisaid system toreduce the amplitude of the sample of said maximum voltage point to the value of the am lplitude of the sample of said minimum voltage :n point of a standing wave in said system, extracting and measuring the value of a proportional measuring the attenuation required to effect said reduction as a measure of the standing wave ratios.

12. A method of determining standing wave ratios in an ultra-high frequency system which comprises extracting and measuring the value of a proportional sample of the power at a minimum voltage point of a standing wave in said system, extracting and measuring the value of a proportional sample of the power at the maximum voltage point of said standing wave, attenuating power in said system to reduce the amplitude of the sample of power from said maximum voltage ratios in an ultra-high frequency system which;

comprises extracting and determining the value of a proportional sample of the minimum voltage point of astanding Wave in said system, extracting and determining the value of a proportional sample of the maximum voltage point of said sample of the maximum voltage point ofl said standing wave, attenuating power in said system to reduce the amplitude o1' the sample of said maximum voltage point to the value of the amplitude of the sample of said minimum voltage point, and measuring the attenuation required to effect said reduction as a measure of the standing wave ratio.

l5. A method of determining electrical characteristics in an ultra-high frequency system which comprises propagating electromagnetic waves through said system, extracting and determining the value of a proportional sample of the minimum voltage point of la standing wave in said system, extracting and determining the value of a proportional sample ofthe maximum voltage point of said standing wave, attenuating said sample of the maximum voltage point to reduce the amplitude of the sample of said maximum voltage point to the value of the amplitude of the sample of said minimum voltage point, and measuring the attenuation required to effect said reduction as a measure of said characteristics. y

16. A method of determining electrical characteristics` in an ultra-high frequency system which comprises propagating electromagnetic waves through said system, extracting and measuring a proportional sample of the power at a minimum voltage point of a standing wave in said system, extracting and measuring a proportional sample of the power at the maximum voltage point of saidstandixig wave, attenuating said sample of power from the maximum voltage point to reduce the amplitude of the sample of power from said maximum voltage point to the value 'of the amplitude of the sample of power from said minimum voltage point, and measuring the attenuation required to effect said reduction as a measure of -said electrical characteristics.

17. A method of determining electrical characteristics in an ultra-high frequency system which comprises propagating electromagnetic waves through said system, extracting and measuring the value of a proportional sample of the minimum voltage point of a standing wave in said system, extracting and measuring the value of a proportional sample of the maximum voltage point of said standing wave, -modifying said sample of the maximum voltage point by attenuation to reduce the amplitude of the sample of said maximum voltage point to the value of the amplitude of the sample of said minimum voltage point, and measuring the attenuation re-` quired to effect said reduction as a measure of said electrical characteristics.

18. A method of determining electrical characteristics in an ultra-high frequency system Y 2,580,248 l1 f l2 said system. extracting and measuring a pro- UNITED-STATES PATENTS pox-tional sample of the power at the maximum -yoltage point of said standing wave, modifying Nr-blen carme Febllo said sample from the maximum voltage point by 214031289 Kumar; J 2' m Aattenuation to reduce the amplitude of the 5 2404 797 Hansen '3 Yao' his lsample o! power from said maximum voltage 2454'042 Demn e; Nul? 16' u point to the value of the amplitude of the s ample 2468'125 su e g v ze' 19 'of power from said minimum voltage point, and v r Apr' 'measuring the attenuation required to effect OTHER REFERENCES 1d fefiuction as a measure 0f Said electrical 10 Meagneret M ,Practical Analysis 01mm. High characteristics. Frequency, 2d edition, 1943, R. C. A. Service HARRY R. LARSON. Cm' me" pages 2ML REFERENCES CITED The following references are of record in the 15 ille ot this patent: 

